I'll try this over at trannies and stuff but maybe someone here has lowered their truck and ran into a vibration. At about 60mph or more when I let off the gas just a bit I'm getting a serious buzzing vibration. That's the only time I noticed it. It does appear that my rear phase angle is "zero" but would that cause a vibration?

 

Try looking at the info at this link:

http://www.drivetrain.com/driveline_angle_problem.html

Generally both U-joint angles must match and they must have a small angle, -not zero...

 

Mr, Bear Tracks,

My truck is doing the very same thing! It is a '79 F-150 w/302 and 4spd OD tranny and 217k miles and stock suspension.

I changed out the old 2.75 geared center section for one with 3.25 gears three years ago and about a year after that the vibration slowly began to appear. And as you said, anything over about 55 mph if I ease off the throttle to where there is no load on the drivetrain you would think that I was driving on the rumble strip along side some interstate highways. If I step down on the throttle just enough to load the drivetrain, its smooth as glass, or if I back out of the throttle all the way it becomes smooth as glass.

I've checked the rear springs, U-bolts, and spring shakels. All seem tight and unmolested. (BTW, I've owned this truck since new so i know its history.)

I've rotated the tires. I've balanced the tires. (I don't think the problem is in the tires, brake drums, rear axles, or rear axle wheel bearings, as the vibration frequency is just too high as compared to the tire/brake drum/axle rotation speed. I >think< its in the tranny output shaft/driveshaft/pinion shaft.

I've disconnected the driveshaft and rotated it 180 degrees - no help. I've pulled the pinion flange and rotated it 180 degrees - no help. I've replaced the u-joints (originals with 200k miles on 'em - very very slight shinny spots, but nothing you could feel with a finger nail) - no help. I sent the driveshaft out to be checked and rebalanced - no help.

I now suspect the tranny output shaft as there appears to be some sideways movement in the tailshaft bushing, but not enough to cause any oil seepage past the output shaft seal. Nobody seems to publish a spec for the output shaft lateral movement. I was just at a tranny shop yesterday and they didn't think the output bushing would be suspect unless there was tranny gear oil getting past the oil seal.

My next place to look is going to be inside the differential. Anyone know if a worn pinion shaft inside the carrier can cause a vibration?

Sorry to write a novel, but if a-n-y-o-n-e can shed any light on this problem I would like to TIA

 

A zero U-joint angle will wear the joint rapidly. If the angles do not match the driveshaft will try to vibrate and twist imposing large loads on the assy.

 

Also you said your truck is lower'd? That would explaine the Zero angle.... Also does it leak anything, and have you looked underneath to see if there are any worn parts since you lower'd it?

 

ditto

Well it must have something to do with angles because i just put air shocks (in rear and raised truck)on my 75 300 6L 3 speed and it NOW rumbles when I let off gas at speeds of 45-55 mph.....



It feels like its under my seat..I am puzzled as well ....But angle of drive shaft may have something to do with it.....


but in my case i raised truck ......a few inches

Unless this is coincidence but I think not.....

 

Well nice reply's. Here's where I'm at.

I know the 0 phase angle (both ends) will wear the joints out but I have a hard time associating that with the buzzing I get in the gas pedal.The spring shop had my drive shaft shortened and I think they took too much out. I have a good 1 1/2" of slip joint showing at the tail shaft of the tranny. My best guess at this point is the joint isn't in far enough. When loaded (accelerating/decelerating) there's no problem but when I "float" the engine there is no torque to prevent the front of the shaft from wobbling. Everything is pretty new under there.


Tail shaft bushing is good. I once had one that wore out and it would bind when comming to a stop or starting out.

pdq, I think you could get the same effect with a worn bushing.

I plan on using shims to rotate the pinon and one to raise the tranny a little.

I'm going back to the spring shop and run the truck while on jack stands to observe the drive shaft.

Thanks!

 

My truck was lifted and anytime I let off the gas when going over 45MPH there was the buzzing sound (under my seat area) and the vibration. Well it eventually wore the ujoint out and my drive shaft came out doing about 60MPH. Needless to say I now have brand new rear drive shafts. I had to make a spacer to decrease the amount of angle because of the lift kit. When I did this the buzzing sound and vibration went away. The place I had my driveshafts built said to make the angle the same at each ujoint. Hope this helps.

 

That link I listed above explains a lot of these problems very well. They also give nice limitation figures etc for U-Joints. Many times that vibration is caused by the driveshaft twisting when the U-Joints bind. In this case you may have a double problem with that slip joint. The drive shaft bind will try to twist around at that joint also.

 

Torque1st,

Good info on that driveline link you listed above. I'm having a problem following what the "continuous operating angle" is. On the drawing they say that it should be at least 1/2 degree, but I don't see it on the drawing. And I can't figure out how its calculated. Can you enlighten me?

 

Compare the angle say of the pinion flange and the driveshaft itself. The difference in the angles can not be 90. they have to be greater than 90.5 or less than 89.5 depending on how you are figuring.

If the U-Joint bearings do not rotate a little every rotation of the shaft the needle bearings will squish out the lubricant film between the rollers and the races. Without lubrication between the metal surfaces the parts will literally beat each other to death breaking off little metal particles which will act as a grit under the rollers and increase the wear. This will create slop in the joint which just allows the bearing rollers to slam into the races harder.

 

Torque1st

 

Yes?

 

(Sorry about that last post.)

Torque1st

I think I understand why you want at least some angle at the u-joints. BTW, that was a good explanation.

So the "continuous operating angle must be greater than 1/2 degree" is refering to the angle between the tranny output shaft/driveshaft and the angle between the driveshaft/pinion, each of which must be greater than 1/2 a degree?

The " '79 Ford Truck Shop Manual - Body, Chassis & Electrical" lists the driveline angles for various model/wheelbase/spring capacity/engine/transmission combinations. The F-150 (4x2) with 133" wheelbase, 3350# spring capacity, 302 V-8 and 4speed O.D calls for the following angles measured to Horizontal:

Engine angle 5 degrees
Driveshaft angle 7.5 degrees
Pinion angle 6 degrees

That means the front u-joint has an operationl angle of 2.5 degrees, the rear u-joint has an operational angle of 1.5 degrees,
(good cancellations of u-joint operating angles because they are within 1 degree of each other), operating angles are less than 3 degrees, and the u-joints won't chew themselves up because they are not running at 0 degrees.

So, what would the "continuous operating angle" be, and is it less than 1/2 degree?

TIA for taking the time to help me understand this interessting and usefull information.

 

I think I understand why you want at least some angle at the u-joints. BTW, that was a good explanation.

-Thanks

So the "continuous operating angle must be greater than 1/2 degree" is refering to the angle between the tranny output shaft/driveshaft and the angle between the driveshaft/pinion, each of which must be greater than 1/2 a degree?

-Correct

The " '79 Ford Truck Shop Manual - Body, Chassis & Electrical" lists the driveline angles for various model/wheelbase/spring capacity/engine/transmission combinations. The F-150 (4x2) with 133" wheelbase, 3350# spring capacity, 302 V-8 and 4speed O.D calls for the following angles measured to Horizontal:

Engine angle 5 degrees
Driveshaft angle 7.5 degrees
Pinion angle 6 degrees

That means the front u-joint has an operationl angle of 2.5 degrees, the rear u-joint has an operational angle of 1.5 degrees,
(good cancellations of u-joint operating angles because they are within 1 degree of each other), operating angles are less than 3 degrees, and the u-joints won't chew themselves up because they are not running at 0 degrees.

So, what would the "continuous operating angle" be, and is it less than 1/2 degree?

-The continuous operating angle of each joint is:
-Front 2.5 D
-Rear 1.5 D

The reason there is a 1D difference in the pinion angle is that when the vehicle is loaded the springs will deflect causing the rear pinion angle to decrease. Probably going to 5 D and then at heavier loads to 4 D. Of course the transmission and driveshaft angles change also...

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Does this help?